Project Summary Abstract Basal cell carcinoma (BCC), a non-melanoma skin cancer (NMSC) type, is a major health problem in the United States (US); annual BCC incidences alone are higher than all other cancer incidences combined (1.67 million/year). Most BCC cases are curable by surgery/radiation, but these can be painful and highly disfiguring and are not viable treatment options for BCC patients with locally advanced and metastatic disease where chemotherapy has also not proven effective. Since Hedgehog (Hh) signaling plays a central role in BCC development and progression, recently FDA has approved GDC-0449 and LDE-225 (Hh pathway inhibitors) to treat BCC; however, acquired resistance and toxicities are limitations. Taken together, clearly there is an urgent need to develop additional non-toxic strategies towards BCC prevention and intervention. In past ~20 years, we have extensively studied and reported chemopreventive efficacy of silibinin against UVB- induced skin cancer in squamous cell carcinoma (SCC) model of NMSC; currently, we are in advanced stages of developing silibinin formulation for human skin use. Based on findings showing strong silibinin efficacy in mouse skin SCC model, in preliminary studies, we also assessed its effects against UVB-induced BCC. Using Ptch1+/? mouse model of BCC, we observed that silibinin decreases UVB-induced: cyclobutane pyrimidine dimer (CPD) positive cells in ear skin, microscopic BCC formation, dermal mast cell infiltration and bone morphogenetic protein (BMP)-2 gene and protein levels in skin. Furthermore, silibinin decreased: MC9 mast cell chemotactic-migration towards mouse BCC ASZ cells and mucosal mast cell degranulation. Silibinin also decreased proliferation of mouse BCC cells in culture and mouse allografts together with inhibition of mitogenic signaling pathways. In most recent studies, silibinin significantly increased the growth inhibitory effects of Hh pathway inhibitor drugs and reversed the resistance of BCC cells to these drugs via targeting EGFR/AKT and Hh pathway and induction of apoptosis. Based on these highly significant and encouraging findings in BCC models, our hypothesis is that by targeting UVB-induced DNA damage repair and reversal of mast cells-mediated immunosuppressive events, together with modulating signaling pathways associated with drug resistance, silibinin has the potential to be an effective agent for both prevention and adjuvant therapy of BCCs. Our aims are to: I) establish and define the role of silibinin-mediated DNA repair in its preventive efficacy against UVB-induced BCC; II) assess and establish the link between DNA damage repair, mast cells, and reversal of UVB-induced immunosuppression by silibinin in its efficacy against BCC formation; III) further establish the therapeutic efficacy of silibinin against BCC, both alone and in combination with FDA approved anti-BCC therapeutics, and define the associated mechanisms.